With concurrent sweeping finc if modified by runfinq and queuefinalizer concurrently.
Fixes crashes like this one:
http://build.golang.org/log/6ad7b59ef2e93e3c9347eabfb4c4bd66df58fd5a
Fixes #7324.
Update #7396

LGTM=rsc
R=golang-codereviews, minux.ma, rsc
CC=golang-codereviews, khr
https://golang.org/cl/67980043